Secondary battery

ABSTRACT

A secondary battery includes: a positive electrode; a negative electrode; a porous electron-insulating layer adhered to a surface of at least one selected from the group consisting of the positive electrode and the negative electrode; and an electrolyte. The porous electron-insulating layer comprises a particulate filler and a resin binder, and the particulate filler comprises an indefinite-shape particle comprising a plurality of primary particles that are joined to one another. A neck is preferably formed between the primary particles. Since the porous electron-insulating layer has high porosity, it is possible to obtain a secondary battery that exhibits excellent low-temperature characteristics, which are particularly important in actual use, and that is capable of discharging at a large current.

RELATED APPLICATION

This application is a national phase of PCT/JP2005/001762 filed on Feb.7, 2005, which claims priority from Japanese Application No. 2004-041106filed on Feb. 18, 2004, the disclosures of which Applications areincorporated by reference herein. The benefit of the filing and prioritydates of the International and Japanese Applications is respectfullyrequested.

TECHNICAL FIELD

The present invention relates to a secondary battery, and, specifically,to improvements in the discharge characteristics of the secondarybattery due to improvements in a porous electron-insulating layeradhered to an electrode surface.

BACKGROUND ART

Secondary batteries typically include a positive electrode, a negativeelectrode, and a separator sheet interposed therebetween. The separatorsheet performs the functions of electronically insulating the positiveelectrode from the negative electrode, and holding an electrolyte. Forexample, conventional lithium-ion secondary batteries often include amicro-porous film, made of polyolefin, as the separator sheet. Also, aseparator sheet comprising a polyolefin resin and an inorganic powder,or the like has been proposed (see Japanese Laid-Open Patent PublicationNo. Hei 10-50287). Such a separator sheet is usually produced by drawinga resin sheet that is obtained by a molding method, such as extrusion.

Recently, to improve the quality of secondary batteries, there has beena proposal to adhere a porous electron-insulating layer to an electrodesurface (see Japanese Patent No. 3371301). The porouselectron-insulating layer is formed on an electrode surface by applyinga slurry comprising a particulate filler and a resin binder onto theelectrode surface and drying the applied slurry with hot air. Althoughthe porous electron-insulating layer is used as an alternative to aconventional separator sheet in some cases, it is used in combinationwith a conventional separator sheet in other cases.

The slurry comprising a particulate filler and a resin binder is usuallyprepared by mixing a particulate filler and a resin binder with a liquidcomponent, and evenly dispersing the particulate filler in the liquidcomponent by means of a dispersing device, such as a bead mill. Asschematically shown in FIG. 3, a conventional particulate filler iscomposed mainly of spherical or substantially spherical primaryparticles 31, and a plurality of the primary particles 31 gather by weakvan der Waals forces to form an agglomerated particle 30.

Conventionally, in terms of stabilizing the thickness and void ratio(porosity) of the porous electron-insulating layer, efforts have beenmade to break down the agglomeration of primary particles as much aspossible by means of a dispersing device, such as a bead mill, in orderto evenly disperse independent primary particles in a liquid component(see Japanese Laid-Open Patent Publication No. Hei 10-106530 (FIG. 2)).

DISCLOSURE OF THE INVENTION Problem that the Invention is to Solve

When the porous electron-insulating layer is formed by applying a slurryin which mutually independent, spherical or substantially sphericalprimary particles are evenly dispersed onto an electrode surface anddrying it with hot air, short-circuit or other problems in batteryproduction are improved. However, mutually independent primary particlesare likely to be filled into the porous electron-insulating layer athigh densities, so that the porosity of the porous electron-insulatinglayer tends to lower. As a result, such secondary batteries haveproblems in that their high-rate charge/discharge characteristics andcharge/discharge characteristics in a low temperature environment tendto become insufficient.

On the other hand, for example, the power source of cellular phones ornotebook computers requires a considerable degree of high-ratecharge/discharge characteristics and charge/discharge characteristics ina low temperature environment. Thus, application of conventionalsecondary batteries to the power source of such devices becomesdifficult in some cases. Particularly in a low temperature environmentat 0° C. or lower, the charge/discharge characteristics of conventionalsecondary batteries may lower markedly.

In view of the above, an object of the present invention is to improvethe high-rate charge/discharge characteristics andlow-temperature-environment charge/discharge characteristics of asecondary battery in which a porous electron-insulating layer is adheredto an electrode surface to improve battery safety.

Means for Solving the Problem

The present invention relates to a secondary battery including: apositive electrode; a negative electrode; a porous electron-insulatinglayer adhered to a surface of at least one selected from the groupconsisting of the positive electrode and the negative electrode; and anelectrolyte. The porous electron-insulating layer comprises aparticulate filler and a resin binder, and the particulate fillercomprises an indefinite-shape particle comprising a plurality of primaryparticles that are joined to one another. As used herein,“indefinite-shape” refers to shapes having knots, bumps, or bulges basedon the primary particles, that is, for example, shapes like dendrite,grape clusters or coral, unlike shapes that are spherical or egg-shaped,or that are similar to such shapes.

A neck is preferably formed between at least a pair of the primaryparticles that are joined to one another and that form theindefinite-shape particle. Specifically, the indefinite-shape particleis formed by partially melting a plurality of primary particles forbonding, for example, by heat treatment. The neck is formed when primaryparticles are bonded to one another by diffusion. It should be notedthat a particle having a neck that is not clearly discernable due tosufficient progress of diffusion bonding can also be used as theindefinite-shape particle.

Preferably, the indefinite-shape particle has a mean particle size thatis twice or more than twice the mean particle size of the primaryparticles and not more than 10 μm. More preferably, it has a meanparticle size that is three times or more than three times the meanparticle size of the primary particles and not more than 5 μm. Also, theprimary particles preferably have a mean particle size of 0.05 to 1 μm.

The indefinite-shape particle preferably comprises a metal oxide. Inthis case, the particulate filler can further comprise a resin fineparticle, such as a polyethylene fine particle.

The resin binder contained in the porous electron-insulating layerpreferably comprises a polyacrylic acid derivative.

When the present invention is applied to a lithium ion secondarybattery, it is preferred that the positive electrode comprise acomposite lithium oxide and that the negative electrode comprise amaterial capable of charging and discharging lithium. Also, it ispreferred to use a non-aqueous electrolyte comprising a non-aqueoussolvent and a lithium salt dissolved in the non-aqueous solvent as theelectrolyte.

The secondary battery of the present invention can further comprise aseparator sheet independent of both the positive electrode and thenegative electrode. The separator sheet may be a conventional separatorsheet such as a micro-porous film made of polyolefin, without anyparticular limitation.

Effects of the Invention

The indefinite-shape particles according to the present invention eachcomprise a plurality of primary particles that are joined to oneanother. Thus, they do not easily become separated into independentprimary particles, unlike agglomerated particles comprising a pluralityof primary particles that gather by van der Waals forces. The use ofsuch indefinite-shape particles prevents a particulate filler from beingfilled into a porous electron-insulating layer at high densities.Therefore, it becomes possible to easily form a porouselectron-insulating layer with a porosity much higher than theconventional one, thereby enabling a significant improvement in thehigh-rate charge/discharge characteristics andlow-temperature-environment charge/discharge characteristics ofsecondary batteries.

The indefinite-shape particles each comprising a plurality of primaryparticles that are joined to one another have complicatedthree-dimensional structures. Thus, in forming the porouselectron-insulating layer, the interaction of the indefinite-shapeparticles is considered to prevent the particulate filler from beingfilled at high densities.

The indefinite-shape particles each comprising a plurality of primaryparticles that are joined to one another can maintain their shapes witha high probability even if they are subjected to a strong shearing forceby a dispersing device in a step of dispersing them in a liquidcomponent to form a slurry. Hence, a porous electron-insulating layerwith a high porosity can be formed stably.

Also, the present invention can provide a secondary battery that isexcellent in high-rate charge/discharge characteristics,low-temperature-environment charge/discharge characteristics, and safetyat low costs.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of indefinite-shape particles each comprisinga plurality of primary particles that are joined to one anotheraccording to the present invention;

FIG. 2 is a scanning electron microscope (SEM) photo of a porouselectron-insulating layer according to one example of the presentinvention;

FIG. 3 is a schematic view of a conventional particulate filler; and

FIG. 4 is an SEM photo of a porous electron-insulating layer accordingto one comparative example of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

A secondary battery of the present invention includes a positiveelectrode, a negative electrode, a porous electron-insulating layeradhered to a surface of at least one electrode selected from the groupconsisting of the positive electrode and the negative electrode, and anelectrolyte. While the present invention is preferably applied tolithium-ion secondary batteries, it is also applicable to other varioussecondary batteries, for example, alkaline storage batteries.

The present invention includes all the cases where the porouselectron-insulating layer is interposed between the positive electrodeand the negative electrode. Specifically, the present invention includesa case where the porous electron-insulating layer is adhered only to apositive electrode surface, a case where it is adhered only to anegative electrode surface, and a case where it is adhered to both apositive electrode surface and a negative electrode surface. Also, thepresent invention includes a case where the porous electron-insulatinglayer is adhered only to one side of the positive electrode, a casewhere it is adhered to both sides of the positive electrode, a casewhere it is adhered only one side of the negative electrode, and a casewhere it is adhered to both sides of the negative electrode.

The porous electron-insulating layer comprises a particulate filler anda resin binder, and the particulate filler comprises indefinite-shapeparticles each comprising a plurality of primary particles (e.g., about2 to 10 particles, preferably 3 to 5 particles) that are joined to oneanother. FIG. 1 schematically shows exemplary indefinite-shapeparticles. Indefinite-shape particles 10 each comprise a plurality ofprimary particles 11 that are joined to one another, and a neck 12 isformed between a pair of the primary particles that are joined to oneanother. Generally, primary particles are composed of single crystal, sothe indefinite-shape particles 10 are inevitably polycrystallineparticles. That is, the indefinite-shape particles are polycrystallineparticles, and the polycrystalline particles each comprise a pluralityof primary particles that are bonded by diffusion.

The indefinite-shape particles each comprising a plurality of primaryparticles that are joined to one another can be produced, for example,by heating a conventional particulate filler, i.e., a particulate fillercomprising mutually independent primary particles or agglomeratedprimary particles by van der Waals forces, so as to partially melt theprimary particles and bond the primary particles to one another. Theindefinite-shape particles thus obtained are not easily disintegratedinto independent primary particles even upon application of a shearingforce thereto.

It should be noted that even if a mechanical shearing force is appliedonto a conventional particulate filler, it is difficult to join aplurality of primary particles to one another. Also, it has beenconfirmed that even if primary particles are agglomerated by using abinder, the primary particles become separated into independent primaryparticles upon preparation of a slurry.

While the porous electron-insulating layer also has an action similar tothat of a conventional separator sheet, it is largely different instructure from a conventional separator sheet. Unlike a micro-porousfilm that is obtained by drawing a resin sheet or the like, the porouselectron-insulating layer has a structure in which particles of aparticulate filler are bonded together with a resin binder. Therefore,the tensile strength of the porous electron-insulating layer in theplane direction thereof is lower than that of a separator sheet.However, the porous electron-insulating layer is superior in that evenupon exposure to high temperatures, it does not shrink due to heat,unlike a separator sheet. In the event of an internal short-circuit orupon exposure of a battery to high temperatures, the porouselectron-insulating layer has the actions of preventing expansion of theshort-circuit, preventing abnormal heating, and enhancing the safety ofthe secondary battery.

The porous electron-insulating layer has pores through which a suitableamount of non-aqueous electrolyte is passed. In a secondary battery withan electrode having a porous electron-insulating layer adhered to thesurface, the large current behavior thereof in a low temperatureenvironment, for example, the discharge characteristics at a currentvalue of 2 hour rate (2 C) in a 0° C. environment depend on the porosityof the porous electron-insulating layer (the ratio of the pore volume tothe porous electron-insulating layer).

The porosity of the porous electron-insulating layer can be measured,for example, in the following manner.

First, a particulate filler, a resin binder, and a liquid component aremixed together to prepare a slurry of raw materials for a porouselectron-insulating layer. The liquid component is selected asappropriate, depending on the kind, etc., of the resin binder. Forexample, an organic solvent, such as N-methyl-2-pyrrolidone orcyclohexanone, or water can be used. The dispersing device used forpreparing the raw material slurry is preferably a device capable ofapplying a shearing force such that the indefinite-shape particles arenot disintegrated into primary particles. Preferable examples include,but are not limited to, a medialess dispersing device and a bead millwith mild conditions.

The resultant slurry is passed through a filter of a suitable mesh size.It is then applied onto a base material made of, for example, metal foilso as to achieve a predetermined thickness with a doctor blade, followedby drying. The film formed on the base material is thought to have thesame structure as that of the porous electron-insulating layer adheredto the electrode surface. Thus, the porosity of the film formed on thebase material can be regarded as the porosity of the porouselectron-insulating layer.

The porosity (P) of the film formed on the base material can be obtainedbased on the apparent volume (Va) and the true volume (Vt) of the film,i.e., from the calculation formula: P(%)={100×(Va−Vt)}/Va.

The apparent volume Va of the film corresponds to the product (S×T) ofthe thickness (T) of the film and the upper surface area (S) of thefilm. Also, the true volume (Vt) of the film can be calculated from theweight (W) of the film, the true density (Df) of the particulate filler,the true density (Db) of the resin binder, and the weight ratio betweenthe particulate filler and the resin binder in the film.

For example, when the weight ratio between the particulate filler andthe resin binder is x:(1−x), the true volume Vt of the film correspondsto the sum of the true volume (xW/Df) of the particulate filler and thetrue volume {(1−x)W/Db} of the resin binder.

In the case of using the conventional particulate filler as illustratedin FIG. 3, upon the dispersion treatment for preparing a slurry, theagglomerated particle 30 easily becomes separated into independentprimary particles 31. Consequently, the porosity P of the resultantporous electron-insulating layer is usually a low value less than 45%,and it is difficult to form a porous electron-insulating layer with ahigher porosity. Secondary batteries having such a low-porosity porouselectron-insulating layer have insufficient high-rate charge/dischargecharacteristics and low-temperature charge/discharge characteristics.

On the other hand, in the case of using the indefinite-shape particles10 each comprising a plurality of the primary particles 11 that arejoined to one another, as illustrated in FIG. 1, the resultant porouselectron-insulating layer can easily achieve a porosity P of 45% ormore, or further, 50% or more. The achievement of such high porositiesis not dependent on the material of the particulate filler. Therefore,as long as the shape, particle size distribution, etc., of theindefinite-shape particles are essentially the same, the use of any of,for example, titanium oxide (titania), aluminum oxide (alumina),zirconium oxide (zirconia), and tungsten oxide results in achievement ofessentially the same high porosities.

In applying the present invention to lithium-ion secondary batteries,the maximum particle size of the primary particles is preferably 4 μm orless, and more preferably 1 μm or less. If the primary particles of anindefinite-shape particle are not clearly discernable, the thickestparts of the knots of the indefinite-shape particle can be regarded asthe particle size of the primary particles.

If the primary particle size exceeds 4 μm, it may become difficult toensure the porosity of the resultant porous electron-insulating layer orto bend the electrode plate.

The maximum particle size of the primary particles can be determined,for example, by measuring the particle sizes of at least 1000 primaryparticles in an SEM photo or a transmission electron microscope (TEM)photo of indefinite-shape particles and obtaining their maximum value.Also, when indefinite-shape particles are produced by subjecting primaryparticles to a heat-treatment to partially melt them for bonding, themaximum particle size of the raw material primary particles can beregarded as the maximum particle size of the primary particlesconstituting the indefinite-shape particles. This is because such a heattreatment just for promoting the diffusion bonding of the primaryparticles hardly changes the particle size of the primary particles. Themaximum particle size of the raw material primary particles can bemeasured, for example, by a wet laser particle size distributionanalyzer available from Microtrack Inc.

In measuring the particle size distribution of raw material primaryparticles of indefinite-shape particles by a wet laser particle sizedistribution analyzer or the like, the volume-basis 99% value (D₉₉) ofthe primary particles can be regarded as the maximum particle size ofthe primary particles.

The mean particle size of the primary particles can also be measured inthe above manner. That is, for example, using an SEM photo or atransmission electron microscope (TEM) photo of indefinite-shapeparticles, the particle sizes of at least 1000 primary particles aremeasured, and then their average value is obtained. Alternatively, theparticle size distribution of raw material primary particles ofindefinite-shape particles is measured by a wet laser particle sizedistribution analyzer or the like, and the volume basis 50% value(median value: D₅₀) of the primary particles can be regarded as the meanparticle size of the primary particles.

The mean particle size of the indefinite-shape particles is desirablytwice or more than twice the mean particle size of the primary particles(preferably 0.05 μm to 1 μm) and not more than 10 μm. Also, from theviewpoint of obtaining a stable porous electron-insulating layer that iscapable of maintaining a high porosity over a long period of time, themean particle size of the indefinite-shape particles is more preferablythree times or more than three times the mean particle size of theprimary particles and not more than 5 μm.

The mean particle size of the indefinite-shape particles can bemeasured, for example, by a wet laser particle size distributionanalyzer available from Microtrack Inc. In this case, the volume basis50% value (median value: D₅₀) of indefinite-shape particles can beregarded as the mean particle size of the indefinite-shape particles. Ifthe mean particle size of the indefinite-shape particles is less thantwice the mean particle size of the primary particles, the resultantporous electron-insulating layer may have a closely packed structure. Ifit exceeds 10 μm, the porosity of the resultant porouselectron-insulating layer becomes excessively high (e.g., more than80%), so that its structure may become brittle.

In the case of lithium-ion secondary batteries, the thickness of theporous electron-insulating layer is not to be particularly limited;however, for example, it is desirably 20 μm or less. The raw materialslurry for the porous electron-insulating layer is applied onto anelectrode surface by a die nozzle method, a blade method, or the like.In applying the raw material slurry for the porous electron-insulatinglayer onto an electrode surface, if the mean particle size of theindefinite-shape particles is large, large particles are likely to becaught, for example, in the gap between the electrode surface and theblade tip, so that the resultant film may be streaked, thereby resultingin a decrease in production yields. Therefore, in terms of productionyields, also, the mean particle size of the indefinite-shape particlesis desirably 10 μm or less.

In the present invention, primary particles of a metal oxide arepreferably used to produce indefinite-shape particles. Exemplary metaloxides forming the particulate filler include titanium oxide, aluminumoxide, zirconium oxide, tungsten oxide, zinc oxide, magnesium oxide, andsilicon oxide. They may be used singly or in combination with two ormore of them. Among them, particularly in terms of chemical stability,aluminum oxide (alumina) is preferable, and α-alumina is particularlypreferable.

The particulate filler can contain resin fine particles. Since resinfine particles have a specific gravity of approximately 1.1 and aretherefore much more lightweight than metal oxides having a specificgravity of around 4, they are effective in reducing the weight ofsecondary batteries. As the resin fine particles, for example,polyethylene fine particles can be used.

However, the use of resin fine particles increases production costs.Hence, in terms of production costs, it is desired to use a metal oxidealone, or, in the case of using resin fine particles, to make the ratioof the resin fine particles to the whole particulate filler 50% or lessby weight.

The material constituting the resin binder contained in the porouselectron-insulating layer is not to be particularly limited, andexamples include polyacrylic acid derivatives, polyvinylidene fluoride(PVDF), polyethylene, styrene-butadiene rubber, polytetrafluoroethylene(PTFE), and tetrafluoroethylene-hexafluoropropylene copolymer (FEP).They may be used singly or in combination with two or more of them.

Alkaline storage batteries and lithium-ion secondary batteriespredominantly use an electrode group obtained by winding a positiveelectrode and a negative electrode. However, in order to form such anelectrode group having a wound structure, the porous electron-insulatinglayer to be adhered to the electrode surface needs to be flexible. Fromthe viewpoint of imparting such flexibility to the porouselectron-insulating layer, it is desirable to use a polyacrylic acidderivative as the resin binder.

In the porous electron-insulating layer, the ratio of the resin binderto the total of the particulate filler and the resin binder ispreferably 1 to 10% by weight, and more preferably 2 to 4% by weight.

In applying the present invention to lithium-ion secondary batteries, itis preferred that the positive electrode comprise a composite lithiumoxide, that the negative electrode comprise a material capable ofcharging and discharging lithium, and that the electrolyte comprise anon-aqueous electrolyte including a non-aqueous solvent and a lithiumsalt dissolved therein.

As the composite lithium oxide, for example, a lithium-containingtransition metal oxide, such as lithium cobaltate, lithium nickelate, orlithium manganate, is preferably used. Also, a modifiedlithium-containing transition metal oxide in which a part of thetransition metal is replaced with another element is preferably used.For example, the cobalt of lithium cobaltate is preferably replaced withan element such as aluminum or magnesium, and the nickel of lithiumcobaltate is preferably replaced with cobalt. Such composite lithiumoxides may be used alone or in combination with two or more of them.

Exemplary negative electrode materials capable of charging anddischarging lithium include various natural graphites, variousartificial graphites, silicon composite materials, and various alloymaterials. These materials may be used singly or in combination with twoor more of them.

With respect to the non-aqueous solvent, there is no particularlimitation, and examples include carbonic acid esters such as ethylenecarbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC),diethyl carbonate (DEC), and ethyl methyl carbonate (EMC); carboxylicacid esters such as γ-butyrolactone, γ-valerolactone, methyl formate,methyl acetate, and methyl propionate; and ethers such as dimethylether, diethyl ether, and tetrahydrofuran. Such non-aqueous solvents maybe used singly or in combination with two or more of them. Among them,particularly carbonic acid esters are preferably used.

With respect to the lithium salt, there is no particular limitation, andfor example, LiPF₆, LiBF₄, or the like is preferably used. They may beused singly or in combination with two or more of them.

In order to ensure stability upon overcharge, it is preferred to add tothe non-aqueous electrolyte a small amount of an additive that forms asatisfactory film on the positive electrode and/or the negativeelectrode, for example, vinylene carbonate (VC), vinyl ethylenecarbonate (VEC), cyclohexyl benzene (CHB), or the like.

The secondary battery of the present invention can further include aseparator independent of the positive electrode and the negativeelectrode, in addition to the porous electron-insulating layer. As theseparator, a conventional separator sheet such as a polyolefinmicro-porous film can be used without any particular limitation. In thecase of not using a conventional separator sheet, the porouselectron-insulating layer performs the function of the separator. Inthis case, secondary batteries can be provided at low costs.

In the case of not using a conventional separator sheet in lithium-ionsecondary batteries, the thickness of the porous electron-insulatinglayer is preferably 1 to 20 μm. Also, when using a conventionalseparator sheet (preferable thickness: 5 to 20 μm) in addition to theporous electron-insulating layer, the thickness of the porouselectron-insulating layer is preferably 1 to 15 μm.

The separator sheet needs to be composed of a material that is resistantto the environment inside the secondary battery. Typically, amicro-porous film made of a polyolefin resin, such as polyethylene orpolypropylene, is used. The use of a separator sheet makes ashort-circuit unlikely to occur, thereby resulting in improvements insafety and reliability of the secondary battery.

The present invention is hereinafter described by way of examples,regarding the production of a lithium-ion secondary battery using aparticulate filler comprising aluminum oxide. These examples, however,are merely examples of the secondary battery of the present inventionand are not to be construed as limiting in any way the presentinvention.

Example 1

(i) Preparation of Particulate Filler Comprising Indefinite-ShapeParticles

(a) Primary particles of aluminum oxide (alumina) with a mean particlesize of 0.1 μm, which were the raw material of indefinite-shapeparticles, were heated at a temperature of 1100° C. in air for 20minutes, to produce indefinite-shape particles each comprising aplurality of primary particles joined to one another. The size of theresultant indefinite-shape particles was adjusted by using15-mm-diameter alumina balls and a wet ball mill, to produce aparticulate filler A1 with a mean particle size of 0.5 μm.

(b) Primary particles of titanium oxide (titania) with a mean particlesize of 0.2 μm, which were the raw material of indefinite-shapeparticles, were heated at a temperature of 800° C. in air for 20minutes, to produce indefinite-shape particles each comprising aplurality of primary particles joined to one another. The size of theresultant indefinite-shape particles was adjusted in the same manner asthe particulate filler A1, to produce a particulate filler A2 with amean particle size of 0.5 μm.

(c) Primary particles of aluminum oxide (alumina) with a mean particlesize of 0.5 μm were used as a particulate filler B1 as they were.

(d) A mechanical shearing force was applied onto primary particles ofaluminum oxide (alumina) with a mean particle size of 0.1 μm by avibration mill equipped with a 40-mm-diameter alumina bar, to produce aparticulate filler B2 comprising agglomerated particles with a meanparticle size of 0.5 μm.

(e) Primary particles of aluminum oxide (alumina) with a mean particlesize of 0.1 μm were mixed with 4% by weight of polyvinylidene fluoride(PVDF), to produce a particulate filler B3 comprising agglomeratedparticles with a mean particle size of 0.5 μm.

(ii) Preparation of Raw Material Slurry for Porous Electron-InsulatingLayer

(a) 100 parts by weight of the particulate filler A1, 4 parts by weightof a polyacrylic acid derivative (the solid content of BM720H availablefrom Zeon Corporation)(resin binder), and a suitable amount ofN-methyl-2-pyrrolidone (liquid component) were mixed together with astirring device, to form a mixture having a nonvolatile componentcontent of 60% by weight. This mixture was placed into a bead mill withan internal volume of 0.6 L, together with 3-mm-diameter zirconia beadsin an amount corresponding to 80% by volume of the internal volume. Byoperating the bead mill, the particulate filler A1 was evenly dispersedin the liquid component, to form a slurry A1.

(b) Slurries A2, B1, B2 and B3 were prepared in the same manner as theslurry A1, except for the use of the particulate fillers A2, B1, B2 andB3, respectively, instead of the particulate filler A1.

(iii) Porosity of Porous Electron-Insulating Layer

(a) The slurry A1 was applied onto a metal foil with a doctor blade suchthat the resultant film (porous electron-insulating layer A1) had athickness of approximately 20 μm when dried. The apparent volume Va ofthis film was obtained from the thickness (T) of the film and the uppersurface area (S) of the film, and further, the true volume (Vt) of thefilm was calculated from the weight (W) of the film, the true density(Df) of the particulate filler, the true density (Db) of the resinbinder, and the weight ratio between the particulate filler and theresin binder in the film.

The porosity (P) of the film was obtained from the calculation formula:P(%)={100×(Va−Vt)}/Va, and it was 60%. Also, the film, i.e., the uppersurface of the porous electron-insulating layer A1 was observed throughan SEM, and an SEM photo as shown in FIG. 2 was taken. FIG. 2 revealsthat large pores are formed in the porous electron-insulating layer A1that is filled with indefinite-shape particles comprising a plurality ofprimary particles joined to one another.

(b) Porous electron-insulating layers A2, B1, B2 and B3 were formed byusing the slurries A2, B1, B2 and B3, respectively, instead of theslurry A1, and their porosities were determined. The porosities of theporous electron-insulating layers A2, B1, B2 and B3 were 58%, 44%, 45%and 44%, respectively. Also, the upper surface of the porouselectron-insulating layer B1 was observed through an SEM, and an SEMphoto as shown in FIG. 4 was taken. FIG. 4 reveals that the porouselectron-insulating layer B1 is filled with independent primaryparticles at a high density and that large pores are not formed therein.

Table 1 shows the results.

TABLE 1 Porous Primary Secondary electron- particle particle insulatingsize size* Porosity layer Particulate filler (μm) (μm) (%) A1 Aluminadendrite 0.1 0.5 60 particles A2 Titania dendrite 0.1 0.5 58 particlesB1 Alumina primary 0.5 — 44 particles B2 Alumina agglomerated 0.1 0.5 45particles (shearing force) B3 Alumina agglomerated 0.1 0.5 44 particles(addition of PVDF) *Secondary particles: dendrite particles oragglomerated particles

The above results clearly indicate that the porous electron-insulatinglayers using the particulate fillers comprising indefinite-shapeparticles clearly have higher porosities than those using theparticulate fillers comprising independent primary particles oragglomerated particles. They also indicate that the degree of porosityis hardly influenced by the material constituting the particulate filler(kind of oxides).

An SEM photo has confirmed that in the case of using the particulatefiller B2 comprising the primary particles that were agglomerated by theapplication of the mechanical shearing force with the vibration mill,the porosity of the porous electron-insulating layer is low and that theagglomerated particles became disintegrated into primary particles inthe porous electron-insulating layer B2. Also, an SEM photo hasconfirmed that in the case of the particulate filler B3 comprising theprimary particles that were agglomerated by the addition of PVDF, theagglomerated particles also became disintegrated into primary particlesin the porous electron-insulating layer B3. The reason is probably thatupon the preparation of the slurry, the agglomerated particles aresubjected to the shearing force in the bead mill.

On the other hand, it can be understood that even if theindefinite-shape particles are subjected to the shearing force in thebead mill, they can substantially maintain their shapes without causingthe primary particles to get separated, and that for this reason thehigh porosities were achieved.

(iv) Production of Lithium-Ion Secondary Battery

Using the slurries A1, A2 and B1, lithium-ion secondary batteries with aporous electron-insulating layer adhered to each side of the negativeelectrode were produced, and their charge/discharge characteristics wereevaluated.

(Negative Electrode Production)

3 kg of artificial graphite (negative electrode active material), 75 gof rubber particles composed of a styrene-butadiene copolymer (negativeelectrode binder), 30 g of carboxymethyl cellulose (CMC: thickener), anda suitable amount of water were stirred by a double-arm kneader, to forma negative electrode mixture paste. This paste was applied onto bothsides of a 10-μm thick copper foil and then dried, to produce a negativeelectrode sheet. This negative electrode sheet was rolled such that thetotal thickness was 180 μm and the active material layer density was 1.4g/cm³.

(Positive Electrode Production)

3 kg of lithium cobaltate (LiCoO₂: positive electrode active material),120 g of PVDF (positive electrode binder), 90 g of acetylene black(positive electrode conductive agent), and a suitable amount ofN-methyl-2-pyrrolidone (NMP) were stirred by a double-arm kneader, toproduce a positive electrode mixture paste. This paste was applied ontoboth sides of a 15-μm thick aluminum foil and then dried, to form apositive electrode sheet. This positive electrode sheet was rolled suchthat the total thickness was 160 μm and the active material layerdensity was 3.3 g/cm³.

(Battery Assembly)

The slurry A1 was applied onto both sides of the rolled negativeelectrode sheet and then dried, to form 5-μm thick porouselectron-insulating layers. Thereafter, the negative electrode sheetwith the porous electron-insulating layers adhered to both sides thereofwas cut into a predetermined width such that it could be inserted into acylindrical battery can of size 18650, to produce a negative electrodeof predetermined size. Also, the rolled positive electrode sheet was cutinto a predetermined width such that it could be inserted into acylindrical battery can of size 18650, to produce a positive electrodeof predetermined size.

The positive electrode and the negative electrode with the porouselectron-insulating layers adhered to both sides thereof were wound witha 15-μm thick separator sheet made of polyethylene resin interposedtherebetween, to form an electrode group. This electrode group wasinserted into the cylindrical battery can of size 18650, and 5 g of anon-aqueous electrolyte was injected therein.

The non-aqueous electrolyte used was prepared by dissolving LiPF₆ at aconcentration of 1 mol/L in a solvent mixture of ethylene carbonate,dimethyl carbonate, and ethyl methyl carbonate in a volume ratio of2:3:3 and further dissolving 3% by weight of vinylene carbonate.

Thereafter, the battery can was sealed, to complete a lithium-ionsecondary battery A1 with a design capacity of 2000 mAh. Also,lithium-ion secondary batteries A2 and B1 were produced in the samemanner as the battery A1, except for the use of the slurries A2 and B1,respectively, instead of the slurry A1.

(v) Battery Evaluation

The lithium-ion secondary batteries A1, A2 and B1 thus produced weresubjected to a preliminary charge/discharge twice and stored in a 45° C.environment for 7 days. Thereafter, they were charged and discharged ina 20° C. environment in the following conditions.

(1) Constant current discharge: 400 mA (cut-off voltage 3 V)

(2) Constant current charge: 1400 mA (cut-off voltage 4.2 V)

(3) Constant voltage charge: 4.2 V (cut-off current 100 mA)

(4) Constant current discharge: 400 mA (cut-off voltage 3 V)

(5) Constant current charge: 1400 mA (cut-off voltage 4.2 V)

(6) Constant voltage charge: 4.2 V (cut-off current 100 mA)

After the above-mentioned charges and discharges, the respectivebatteries were allowed to stand for 3 hours. Thereafter, they werecharged and discharged in a 0° C. environment as follows.

(7) Constant current discharge: 4000 mA (cut-off voltage 3 V)

The discharge capacities obtained were designated as 0° C.-2 C dischargecharacteristics. Table 2 shows the 0° C.-2 C discharge characteristicsof the respective batteries.

TABLE 2 0° C.-2° C. discharge Slurry Particulate filler characteristics(mAh) A1 Alumina dendrite 1820 particles A2 Titania dendrite 1780particles B1 Alumina primary 1590 particles

The above results indicate that the batteries with the high-porosityporous electron-insulating layers exhibit excellent low-temperaturedischarge characteristics. On the other hand, the battery with thelow-porosity porous electron-insulating layers having noindefinite-shape particles exhibits a significant decrease inlow-temperature discharge characteristics.

Example 2

The slurry prepared is subjected to a step of refinement by a filter andthe like before being used for forming a porous electron-insulatinglayer. Thus, in order to stabilize the physical properties of the porouselectron-insulating layer, it is desired that the sedimentation of theparticulate filler hardly proceeds for several days after thepreparation of the slurry. The present inventors have found that thedegree of sedimentation is dependent on the mean particle size ofprimary particles of a particulate filler. Hence, this example explainsthe relationship between the mean particle size of primary particles ofa particulate filler and the degree of sedimentation.

Indefinite-shape particles comprising a plurality of primary particlesjoined to one another were produced in the same manner as theparticulate filler A1 of Example 1, except that the mean particle sizeof primary particles of aluminum oxide as the raw material of theindefinite-shape particles was changed from 0.1 μm to 0.01 μm, 0.05 μm,0.3 μm, 0.5 μm, 1 μm, 2 μm, 3 μm or 4 μm. The size of the resultantindefinite-shape particles was adjusted to produce particulate fillersC1, C2, C3, C4, C5, C6, C7 and C8 with mean particle sizes of 0.03 μm,0.16 μm, 0.8 μm, 1.3 μm, 2.5 μm, 6 μm, 8 μm and 10 μm, respectively.

Slurries C1, C2, C3, C4, C5, C6, C7 and C8 were prepared in the samemanner as the slurry A1, except for the use of the particulate fillersC1, C2, C3, C4, C5, C6, C7 and C8, respectively, instead of theparticulate filler A1. The resultant slurries were allowed to stand in a25° C. environment, and the degree of sedimentation was visuallyobserved every day.

As a result, in the slurries C1 to C5 having the mean particle sizes ofprimary particles of 0.01 to 1 μm, little sedimentation of theparticulate filler was found after 4 days or more. On the other hand, inthe slurries C6 to C8 having the mean particle sizes of primaryparticles of 1 μm or more, sedimentation of the particulate filler wasobserved within 1 day.

The particle size distributions of the primary particles used as the rawmaterials of the particulate fillers C1 to C5 were measured, and theirmaximum particle sizes (D₉₉) were all found to be 3 μm or less. On theother hand, the particle size distributions of the primary particlesused as the raw materials of the particulate fillers C6 to C8 weremeasured, and their maximum particle sizes (D₉₉) were all found to belarger than 3 μm.

Lithium-ion secondary batteries C1 to C5 were produced in the samemanner as in the Example 1, except for the use of the slurries C1 to C5,respectively, and their 0° C.-2 C discharge characteristics wereevaluated. As a result, they were all found to be 1750 mAh or more,being significantly better than the battery B1.

Example 3

Indefinite-shape particles comprising a plurality of primary particlesjoined to one another were produced in the same manner as theparticulate filler A1 of Example 1, except that the mean particle sizeof primary particles of aluminum oxide used as the raw material of theindefinite-shape particles was changed from 0.1 μm to 0.2 μm. However,the size of the indefinite-shape particles was varied, to produceparticulate fillers D1, D2, D3, D4, D5, D6, D7, D8, D9 and D10 with meanparticle sizes of 0.3 μm, 0.4 μm, 0.5 μm, 0.7 μm, 1.2 μm, 3 μm, 8 μm, 10μm, 12 μm and 15 μm, respectively.

The size of the indefinite-shape particles was adjusted by using a wetball mill equipped with 3-mm-diameter alumina balls occupying 30% of theinternal volume. The mean particle size of the indefinite-shapeparticles was varied by varying the operation time of the ball mill.

Slurries D1, D2, D3, D4, D5, D6, D7, D8, D9 and D10 were prepared in thesame manner as the slurry A1, except for the use of the particulatefillers D1, D2, D3, D4, D5, D6, D7, D8, D9 and D10, respectively,instead of the particulate filler A1.

Each slurry was applied onto a metal foil with a common blade coater.The intended thickness of the film was 20 μm. As a result, the slurriesD1 to D8, in which the mean particle sizes of the indefinite-shapeparticles were 10 μm or less, resulted in films having a smooth surface.On the other hand, in the case of using the slurries D9 and D10, inwhich the mean particle sizes of the indefinite-shape particles werelarger than 10 μm, the surfaces of the resultant films were streakedwith a high frequency, thereby leading to a large decline in productionyields.

Next, lithium-ion secondary batteries D1 to D8 were produced in the samemanner as in Example 1, except for the use of the slurries D1 to D8,respectively, and their 0° C.-2 C discharge characteristics wereevaluated. As a result, they were all found to be 1750 mAh or more,being significantly superior to the battery B1.

Also, with respect to the 0° C.-2 C discharge characteristics, theresults of the batteries D2 to D5 were particularly good, and theresults of the batteries D3 and D4 were more particularly good.Accordingly, the mean particle size of the indefinite-shape particles ispreferably about 2 to 6 times the mean particle size of the primaryparticles, and most preferably about 2.5 to 3.5 times.

Example 4

The most common structure of the electrode group of secondary batteriesis a structure of winding a positive electrode and a negative electrodewith a separator sheet therebetween. Thus, if the porouselectron-insulating layer is hard, the porous electron-insulating layermay become cracked or separated from the electrode surface. Hence, thisexample describes the case where the flexibility of the porouselectron-insulating layer is changed.

A slurry E1 was prepared in the same manner as the slurry A1 of Example1, except for the use of polyvinylidene fluoride (PVDF) instead of thepolyacrylic acid derivative. Using this slurry, 500 batteries wereproduced in the same manner as the battery A1 of Example 1. Also, 500batteries A1 of Example 1 were produced.

The inter-terminal voltages of the respective batteries were measured tocheck the presence or absence of an internal short-circuit. As a result,the short-circuit incidence of the batteries E1 was 10%, which was high,but the short-circuit incidence of the batteries A1 was 0.4%. Theshort-circuit incidence of conventional lithium-ion secondary batteriesis 0.5% or less. The short-circuited batteries E1 were disassembled, andthe states of their porous electron-insulating layers containing PVDF asthe resin binder were observed. As a result, many cracks were found, andcracking particularly near the center of the electrode group wasremarkable.

On the other hand, the batteries A1 were disassembled, and the states oftheir porous electron-insulating layers containing the polyacrylic acidderivative as the resin binder were observed. As a result, no crackingwas found. The above results indicate that while PVDF yields relativelyhard porous electron-insulating layers, the polyacrylic acid derivativeyields highly flexible porous electron-insulating layers.

Example 5

A battery F1 was produced in the same manner as the battery A1 ofExample 1, except that the thickness of the porous electron-insulatinglayer on each side of the negative electrode was changed from 5 μm to 20μm and that the polyethylene resin separator sheet was not used.

The 0° C.-2 C discharge characteristics of the battery F1 wereevaluated, and it was found that 1750 mAh was achieved. This confirmedthat the battery F1 had excellent low-temperature dischargecharacteristics equivalent to those of battery A1. It should be notedthat the battery F1 does not need a conventional expensive separatorsheet, thereby making the production cost low, which holds greatindustrial promise.

INDUSTRIAL APPLICABILITY

The present invention is applicable to various secondary batteriesincluding lithium ion secondary batteries, and is particularly effectivein lithium ion secondary batteries that require both highlow-temperature discharge characteristics and high-rate dischargecharacteristics as well as high safety. Since lithium ion secondarybatteries have a large market, the present invention can play asignificant role in improving product performance and safety.

1. A secondary battery comprising: a positive electrode; a negativeelectrode; a porous electron-insulating layer adhered to a surface of atleast one selected from the group consisting of said positive electrodeand said negative electrode; and an electrolyte, wherein said porouselectron-insulating layer comprises a particulate filler and a resinbinder, and said particulate filler substantially comprises anindefinite-shape particle, comprising a plurality of single crystallineparticles, which has the shape of dendrites, grape clusters, or coral,said shape having a neck, wherein said neck is formed between at least apair of said single crystalline particles, said neck comprising the samematerial as said single crystalline particles, and wherein saidindefinite-shape particle comprises a metal oxide.
 2. The secondarybattery in accordance with claim 1, wherein said indefinite-shapeparticle comprises a plurality of primary particles bonded to eachother, and said indefinite-shape particle has a mean particle size thatis twice or more than the mean particle size of said primary particlesand not more than 10 μm.
 3. The secondary battery in accordance withclaim 1, wherein said particulate filler further comprises resinparticles.
 4. The secondary battery in accordance with claim 1, whereinsaid positive electrode comprises a composite lithium oxide, saidnegative electrode comprises a material capable of charging anddischarging lithium, and said electrolyte comprises a non-aqueoussolvent and a lithium salt dissolved in the non-aqueous solvent.
 5. Thesecondary battery in accordance with claim 1, further comprising aseparator sheet that is interposed between said positive electrode andsaid negative electrode, said separator sheet being independent of bothsaid positive electrode and said negative electrode.
 6. A secondarybattery comprising: a positive electrode; a negative electrode; a porouselectron-insulating layer adhered to a surface of at least one selectedfrom the group consisting of said positive electrode and said negativeelectrode; and an electrolyte, wherein said porous electron-insulatinglayer comprises a particulate filler and a resin binder, and saidparticulate filler substantially comprises indefinite-shape particles,wherein said indefinite-shape particles are polycrystalline particlescomprising a plurality of single crystalline particles that arediffusion bonded to each other, and a neck is formed between at least apair of said single crystalline particles, said neck comprising the samematerial as said single crystalline particles, wherein said porouselectron-insulating layer has a porosity of 50% or more, saidindefinite-shape particles comprise a metal oxide.
 7. The secondarybattery in accordance with claim 6, wherein said indefinite-shapeparticles have a mean particle size that is twice or more than the meanparticle size of said single crystalline particles and not more than 10μm.
 8. The secondary battery in accordance with claim 6, wherein saidparticulate filler further comprises resin particles.
 9. The secondarybattery in accordance with claim 1, wherein said indefinite-shapeparticles are adhered with said binder.
 10. The secondary battery inaccordance with claim 6, wherein said indefinite-shape particles areadhered with said binder.
 11. The secondary battery in accordance withclaim 1, wherein said metal oxide comprises alumina particles.
 12. Thesecondary battery in accordance with claim 6, wherein said metal oxidecomprises alumina particles.